Polymer membrane, method for the production and use thereof

ABSTRACT

The present invention relates to an acid-doped polymer membrane based on polyazoles. The acid-doped polymer membrane can be used in a variety of applications because of its excellent mechanical properties and is useful as polymer electrolyte membrane (PEM) in PEM fuel cells. A doped polymer membrane based on polyazoles is obtained by a process comprising the steps of: A) casting a film using a solution of polymers based on polyazoles in a polar, aprotic organic solvent; B) drying the film formed in step A) until it is self-supporting; C) treating the film obtained in step B) with a treatment liquid at a temperature in the range from room temperature to the boiling point of the treatment liquid; D) drying and/or dabbing the film treated according to step C) to remove the treatment liquid from step C); and E) doping the film treated according to step D) with a doping agent.

RELATED APPLICATIONS

This case is a continuation of U.S. patent application Ser. No.10/468,385.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will become more readily apparent by referring to thefollowing detailed description and the appended drawings in which:

FIG. 1 is a graph showing the results of the KF filtration;

FIG. 2 is a graph showing proton conductivity;

FIG. 3 is a graph showing the results of tensile strength on the polymermembranes.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an acid-doped polymer membrane based onpolyazoles, a process for producing it and its use.

The acid-doped polymer membrane of the invention can be used in avariety of applications because of its excellent chemical, thermal andmechanical properties and is particularly useful as polymer electrolytemembrane (PEM) in PEM fuel cells.

Acid-doped polyazole membranes for use in PEM fuel cells are alreadyknown. The basic polyazole membranes are doped with concentratedphosphoric acid or sulfuric acid and act as proton conductors andseparators in polymer electrolyte membrane fuel cells (PEM fuel cells).

Due to the excellent properties of the polyazole polymer, such polymerelectrolyte membranes can, when processed to produce amembrane-electrode unit (MEE), be used in fuel cells at continuousoperating temperatures above 100° C., in particular above 120° C. Thishigh continuous operating temperature allows the activity of thecatalysts based on noble metals present in the membrane-electrode unit(MEE) to be increased. Particularly when using reformates fromhydrocarbons, significant amounts of carbon monoxide are present in thereformer gas and these usually have to be removed by costly gastreatment or gas purification procedures. The opportunity of increasingthe operating temperature enables significantly higher concentrations ofCO impurities to be tolerated over the long term.

The use of polymer electrolyte membranes based on polyazole polymersenables, firstly, part of the costly gas treatment or gas purificationprocedures to be omitted and, secondly, the catalyst loading in themembrane electrode unit to be reduced. Both are indispensableprerequisites for large-scale use of PEM fuel cells, since otherwise thecosts of a PEM fuel cell system are too high.

The acid-doped, polyazole-based polymer membranes known hitherto displaya favorable property profile. However, owing to the applications soughtfor PEM fuel cells, in particular in automobile and stationaryapplications, they still require overall improvement. Furthermore, thepolymer membranes known hitherto have a high content ofdimethylacetamide (DMAc) which cannot be removed completely by knowndrying methods.

Thus, the polyazole-based polymer membranes known hitherto still displaymechanical properties which are unsatisfactory for the above applicationafter they have been doped with acid. This mechanical instability isreflected in a low modulus of elasticity, a low ultimate tensilestrength and a low fracture toughness.

It is an object of the present invention to provide acid-dopedpolymembranes based on polyazoles which, firstly, have improvedmechanical properties and, secondly, have the advantages of the polymermembrane based on polyazoles and allow an operating temperature above100° C. without additional humidification of the combustion gas.

We have now found that a specific after-treatment of the polyazole-basedfilm to be doped with acid surprisingly leads to doped polymer membraneshaving improved mechanical properties, with the good proton conductivitybeing retained or even improved. In addition, the after-treatment ridsthe membrane of residual organic constituents such as dimethylacetamide(DMAc) which would otherwise reduce the catalyst activity.

The present invention provides a doped polymer membrane based onpolyazoles, obtainable by a process comprising the steps

A) casting a film using a solution of polymers based on polyazoles in apolar, aprotic organic solvent,

B) drying the film formed in step A) until it is self-supporting,

C) treating the film obtained in step B) with a treatment liquid at atemperature in the range from room temperature to the boiling point ofthe treatment liquid,

D) drying and/or dabbing the film treated according to step C) to removethe treatment liquid from step C),

E) doping the film treated according to step D) with a doping agent.

The preparation of polymer solutions based on polyazoles has beencomprehensively described in the prior art. Thus, EP-A-0816415 describesa method of dissolving polymers based on polyazoles usingN,N-dimethylacetamide as polar, aprotic solvent at temperatures above260° C. A substantially more gentle process for preparing solutionsbased on polyazoles is disclosed in the German patent application10052237.8.

As polymers based on polyazoles, preference is given to polymerscomprising recurring azole units of the formula (I) and/or (II)

where

Ar are identical or different and are each a tetravalent aromatic orheteroaromatic group which may have one or more rings,

Ar¹ are identical or different and are each a divalent aromatic orheteroaromatic group which may have one or more rings,

Ar² are identical or different and are each a trivalent aromatic orheteroaromatic group which may have one or more rings,

X are identical or different and are each oxygen, sulfur or an aminogroup which bears a hydrogen atom and a group having 1-20 carbon atoms,preferably a branched or unbranched alkyl or alkoxy group, or an arylgroup as other radical.

Preferred aromatic or heteroaromatic groups are derived from benzene,naphthalene, biphenyl, diphenyl ether, diphenylmethane,diphenyldimethylmethane, bisphenone, diphenyl sulfone, quinoline,pyridine, bipyridine, anthracene and phenanthrene, all of which may alsobe substituted.

Ar¹ can have any substitution pattern; in the case of phenylene, forexample, Ar¹ can be ortho-, meta- or para-phenylene. Particularlypreferred groups are derived from benzene and biphenylene, each of whichmay also be substituted.

Preferred alkyl groups are short-chain alkyl groups having from 1 to 4carbon atoms, e.g. methyl, ethyl, n-propyl or isopropyl and tert-butylgroups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups may be substituted.

Preferred substituents are halogen atoms such as fluorine, amino groupsor short-chain alkyl groups such as methyl or ethyl groups.

If polyazoles comprising recurring units of the formula (I) are used forthe purposes of the present invention, the radicals X within a recurringunit should be identical.

The polyazoles used according to the invention can in principle alsohave different recurring units which differ, for example, in theirradical X. However, they preferably have only identical radicals X in arecurring unit.

In a preferred embodiment of the present invention, the polymercomprising recurring azole units is a copolymer comprising at least twounits of the formula (I) and/or (II) which differ from one another.

In a particularly preferred embodiment of the present invention, thepolymer comprising recurring azole units is a polyazole comprising onlyunits of the formula (I) and/or (II).

The number of recurring azole units in the polymer is preferably greaterthan or equal to 10. Particularly preferred polymers comprise at leastone 100 recurring azole units. For the purposes of the presentinvention, polymers comprising recurring benzimidazole units arepreferably used. An example of an extremely advantageous polymercomprising recurring benzimidazole units is represented by the formula(III):

where n is an integer greater than or equal to 10, preferably greaterthan or equal to 100.

Casting of a polymer film from a polymer solution according to step A)is carried out by means of measures known per se from the prior art.

Drying of the film in step B) is carried out at temperatures in therange from room temperature to 300° C. Drying is carried out underatmospheric pressure or reduced pressure. The drying time depends on thethickness of the film and is preferably from 10 seconds to 24 hours. Thefilm which has been dried in step B) is subsequently self-supporting andcan be processed further. Drying is carried out by means of dryingmethods customary in the films industry.

As a result of the drying procedure carried out in step B), the polar,aprotic organic solvent is very largely removed. Thus, the residualcontent of the polar, aprotic organic solvent is usually 10-23%.

A further lowering of the residual solvent content to below 2% by weightcan be achieved by increasing the drying temperature and drying time,but this significantly prolongs the subsequent doping of the film, forexample with phosphoric acid. A residual solvent content of 5-15% isthus useful for reducing the doping time.

The treatment of the film which has been dried in step B) uses atreatment liquid and is out in the temperature range from roomtemperature (20° C.) and the boiling point of the treatment liquid atatmospheric pressure.

As treatment liquid for the purposes of the invention and for thepurposes of step C.), use is made of solvents which are liquid at roomtemperature [i.e. 20° C.] selected from the group consisting ofalcohols, ketones, alkanes (aliphatic and cycloaliphatic), ethers(aliphatic and cycloaliphatic), esters, carboxylic acids, with theabovementioned group members being able to be halogenated, water,inorganic acids (e.g. H₃PO₄, H₂SO₄) and mixtures thereof.

Preference is given to using C1-C10 alcohols, C2-C5 ketones,C1-C10-alkanes (aliphatic and cycloaliphatic), C2-C6-ethers (aliphaticand cycloaliphatic), C2-C5 esters, C1-C3 carboxylic acids,dichloromethane, water, inorganic acids (e.g. H₃PO₄, H₂SO₄) and mixturesthereof.

The treatment liquid introduced in step C) can be removed by means ofthe drying procedure carried out in step D). The drying proceduredepends on the partial vapor pressure of the treatment liquid chosen.Drying is usually carried out at atmospheric pressure and temperaturesin the range from 20° C. to 200° C. More gentle drying can also becarried out under reduced pressure. In place of drying, the membrane canalso be dabbed to free it of excess treatment liquid in step D). Theorder is not critical.

In step E), the doping of the film obtained from step C) or D) iscarried out. For this purpose, the film is wetted with a doping agent orlaid in this. As doping agent for the polymer membrane of the invention,use is made of acids, preferably all known Lewis and BrØnsted acids, inparticular inorganic Lewis and BrØnsted acids.

Apart from these abovementioned acids, the use of polyacids, inparticular isopolyacids and heteropolyacids, and of mixtures of variousacids is also possible. For the purposes of the present invention,heteropolyacids are inorganic polyacids which have at least twodifferent central atoms and are in each case partial mixed anhydridesformed from weak, polybasic oxo acids of a metal (preferably Cr, Mo, V,W) and a nonmetal (preferably As, I, P, Se, Si, Te). They include, interalia, 12-molybdophosphoric acid and 12-tungstophosphoric acid.

Doping agents which are particularly preferred for the purposes of theinvention are sulfuric acid and phosphoric acid. A very particularlypreferred doping agent is phosphoric acid (H₃PO₄).

The polymer membranes of the invention are doped. For the purposes ofthe present invention, doped polymer membranes are polymer membraneswhich, owing to the presence of doping agents, display increased protonconductivity compared to the undoped polymer membranes.

Processes for preparing doped polymer membranes are known. In apreferred embodiment of the present invention, they are obtained bywetting a film of the polymer concerned with concentrated acid, forexample with highly concentrated phosphoric acid, for a suitable time,preferably 5 minutes-96 hours, particularly preferably 1-72 hours, attemperatures in the range from room temperature to 100° C. andatmospheric or super-atmospheric pressure.

The conductivity of the polar membrane of the invention can beinfluenced via the degree of doping. The conductivity increases withincreasing concentration of doping agent until a maximum value has beenreached. According to the invention, the degree of doping is reported asmol of acid per mol of recurring units of the polymer. For the purposesof the present invention, a degree of doping of from 3 to 15, inparticular from 6 to 12, is preferred.

The polymer membrane of the invention has improved materials propertiescompared to the previously known doped polymer membranes. In particular,they have very good mechanical properties and perform better thanuntreated membranes.

The polymer membranes of the invention display improved protonconductivity compared to untreated membranes.

Possible applications of the doped polymer membranes of the inventioninclude, inter alia, use in fuel cells, in electrolysis, in capacitorsand in battery systems. Owing to their property profile, the dopedpolymer membranes are preferably used in fuel cells.

The present invention also relates to a membrane-electrode unitcomprising at least one polymer membrane according to the invention. Forfurther information on membrane-electrode units, reference may be madeto the specialist literature, in particular the U.S. Pat. No. 4,191,618,U.S. Pat. No. 4,212,714 and U.S. Pat. No. 4,333,805. The disclosure inthe abovementioned references [U.S. Pat. No. 4,191,618, U.S. Pat. No.4,212,714 and U.S. Pat. No. 4,333,805] in respect of the structure andproduction of membrane-electrode units is incorporated by reference intothe present description.

The invention is illustrated below by means of examples and acomparative example, without the invention being restricted to theseexamples.

EXAMPLES Untreated Film

The untreated films were laid in 85% strength H₃PO₄ for 96 hours. Priorto doping with H₃PO₄, the H₂O content and residual solvent content ofthe film are determined by Karl Fischer (KF) titration. The watercontent of the film is determined directly as follows by KF titrationusing a Mettler-Toledo apparatus. The sample, which is present in aclosed sample vial, is heated to 250° C. and dried at this temperature.The gas liberated in this way is passed directly into a closed titrationvessel and analyzed by means of a Karl Fischer [KF] reagent. Apart fromthe determination of the water content, the residual solvent content isdetermined by determining the weight before and after drying.

Washing with H₂O and Subsequent Thermal Drying:

The films were boiled in water for 1 hour. The water bath was thenchanged and the films were boiled for a further hour. The films weresubsequently rinsed with fresh water and finally dried at 160° C. for 3hours. H₂O content and residual solvent content were determined on thetreated films by KF titration. The membranes were obtained by doping thefilms in 85% strength H₃PO₄ for 96 hours.

Washing with H₂O:

The films were boiled in water for 1 hour. The water bath is thenchanged and the films are boiled for a further hour. The films weresubsequently dabbed with a cloth and used further in moist form. H₂Ocontent and residual solvent content of the film were determined by KFtitration. The membranes were doped in 85% strength H₃PO₄ for 96 hours.

Washing with Methanol:

The films were placed in methanol and boiled under reflux for 2 hours(beginning when the methanol started to boil). The films were taken outand dried firstly for 1 minute in air minute in air and then at 100° C.under reduced pressure in a drying oven for 2 hours. H₂O content andresidual organic solvent content of the film were determined by KFtitration. The membranes were doped in 85% strength H₃PO₄ for 96 hours.

Washing with Acetone:

The films were placed in acetone and boiled under reflux for 2 hours(beginning when the acetone started to boil). The films were then driedfirstly for 1 minute in air at RT and subsequently at 100° C. underreduced pressure in a drying oven for 2 hours. H₂O content and residualsolvent content of the film were determined by KF titration. Themembranes were doped in 85% strength H₃PO₄ for 96 hours.

FIG. 1 shows the result of the KF titration. The residual organicsolvent is removed completely by washing with water. The residualorganic solvent content is reduced from 16.6% to 3.7 or 2.3% by washingwith acetone or with methanol, respectively.

FIG. 2 shows a proton conductivity which is improved by 10% even at roomtemperature and is retained or improved further at elevated temperature.

The specific conductivity is measured by means of impedance spectroscopyin a 4-pole arrangement in the potentiostatic mode using platinumelectrodes (wire, 0.25 mm diameter). The distance between the currentcollector electrodes is 2 cm. The spectrum obtained is evaluated using asimple model consisting of a parallel arrangement of an ohmic resistanceand a capacitor. The specimen cross section of the membrane doped withphosphoric acid is measured immediately before mounting of the specimen.To measure the temperature dependence, the measuring cell is brought tothe desired temperature in an oven and the temperature is regulated viaa Pt-100 temperature sensor positioned in the immediate proximity of thespecimen. After reaching the temperature, the specimen is maintained atthis temperature for 10 minutes before commencement of the measurement.

To determine the mechanical properties, uniaxial tensile tests arecarried out on tension bars. A Zwick tester equipped with a 100 N loadcell and a heatable oven is used for this purpose. The length ofspecimen between the chucks is 10 cm and the separation velocity is setat 50 mm/min. The deformation is determined directly via the distance oftravel. The tensile tests on membranes doped with phosphoric acid arecarried out at 100° C. To calculate the stress automatically, the crosssection of each specimen is determined and entered before commencementof the test. To determine mean values of modulus of elasticity, tensilestrength, elongation at break and rupture energy (toughness), at least 5measurements are carried out on each membrane.

The results of the tensile tests on the polymer membranes according tothe invention compared to untreated membranes are shown by way ofexample in FIG. 3. It can be seen from the figure that a membrane washedwith water has the highest elongation at break and the highest tensilestress at break.

An untreated membrane displays an elongation at break of 55% while amembrane according to the invention has an elongation at break in therange from 58% to 75%.

The results of the tensile tests are summarized in Table 1.

TABLE 1 Results of the tensile tests on membranes after differentwashing procedures compared to an untreated membrane. Error in Error inError in Tensile tensile Elongation elongation Rupture rupture Washing EError in E strength strength at break at break energy energy Method[MPa] [MPa] [MPa] [MPa] [%] [%] [kJ/m²] [kJ/m²] untreated 4.7 0.7 1.50.13 55 5 54 5 washed 5 0.55 1.7 0.25 71 11 74.5 18 with water washed5.45 0.4 1.55 0.14 64.7 6 63 8.8 with acetone washed 5.3 0.5 1.36 0.2261.2 13 54 18.6 with methanol

1. A doped polymer-membrane based on polyazoles, obtained by a processcomprising the steps A) casting a film using a solution of polymersbased on polyazoles in a polar, aprotic organic solvent, B) drying thefilm formed in step A) until it is self-supporting to form a dried film,said dried film having a residual content of said polar, aprotic organicsolvent in a range of 10 to 23%, C) treating the film obtained in stepB) with a treatment liquid at a temperature in the range from 20° C. tothe boiling point of the treatment liquid, D) drying and/or dabbing thefilm treated according to step C) to remove the treatment liquid fromstep C), E) doping the film treated according to step D) with a dopingagent, where said doping agent is selected from the group consisting of:Lewis acids and BrØnsted acids, and where doping is of a degree from 3to 15 mol of acid per mol of recurring unit of polymer.
 2. A polymermembrane as claimed in claim 1 having a degree of doping from 6 to 12mols of acid per mol of recurring units of said polymer.
 3. A polymermembrane as claimed in claim 1, wherein the polymer based on polyazolescomprises recurring azole units of the formula (I) and/or (II)

where Ar are identical or different and are each a tetravalent aromaticor heteroaromatic group which may have one or more rings, Ar¹ areidentical or different and are each a divalent aromatic orheteroaromatic group which may have one or more rings, Ar² are identicalor different and are each a trivalent aromatic or heteroaromatic groupwhich may have one or more rings, X are identical or different and areeach oxygen, sulfur or an amino group which bears a hydrogen atom and agroup having 1-20 carbon atoms, preferably a branched or unbranchedalkyl or alkoxy group, or an aryl group as other radical.
 4. A polymermembrane as claimed in claim 3, wherein the polymer comprising recurringazole units is a copolymer comprising at least two units of the formula(I) and/or (II) which differ from one another.
 5. A polymer membrane asclaimed in claim 4, wherein the polyazole consists only of units of theformula (I) and/or (II).
 6. A polymer membrane as claimed in claim 1,wherein the polyazole is a polymer comprising recurring benzimidazoleunits of the formula (III)

where n is an integer greater than or equal to 10, preferably greaterthan or equal to
 100. 7. A polymer membrane as claimed in claim 1,wherein doping is performed for 1 to 96 hours at a temperature rangingfrom 20° C. to 100° C.
 8. A membrane-electrode unit comprising at leastone polymer membrane as claimed in claim 1 and at least one electrode.9. A polymer electrolyte fuel cell comprising at least onemembrane-electrode unit as claimed in claim
 8. 10. A polymer membrane asclaimed in claim 1 where said doping agent is selected from the groupconsisting of: inorganic Lewis acids and inorganic BrØnsted acids.
 11. Apolymer membrane as claimed in claim 7 where said doping agent isselected from the group consisting of: sulfuric acid and phosphoricacid.
 12. A polymer membrane as claimed in claim 7 where said dopingagent phosphoric acid.
 13. A polymer membrane as claimed in claim 12where said phosphoric acid has a concentration of 85%.
 14. A dopedpolymer membrane based on polyazoles, obtained by a process comprisingthe steps A) casting a film using a solution of polymers based onpolyazoles in a polar, aprotic organic solvent, B) drying the filmformed in step A) until it is self-supporting to form a dried film, saiddried film having a residual content of said polar, aprotic organicsolvent in a range of 10 to 23%, C) treating the film obtained in stepB) with a treatment liquid at a temperature in the range from 20° C. tothe boiling point of the treatment liquid, where said treatment liquidis selected from the group consisting of: water, acetone and methanol,D) drying and/or dabbing the film treated according to step C) to removethe treatment liquid from step C), E) doping the film treated accordingto step D) with a doping agent, where said doping agent is selected fromthe group consisting of: sulfuric acid and phosphoric acid, where saiddoping is performed for 1 to 96 hours at a temperature ranging from 20°C. to 100° C. and said doping agent has a concentration of 85%, andwhere doping is of a degree from 3 to 15 mol of acid per mol ofrecurring unit of polymer.